All-dry flip-over stacking of van der Waals junctions of 2D materials using polyvinyl chloride

We demonstrated an all-dry polymer-to-polymer transfer technique for two-dimensional (2D) crystal flakes using a polyvinyl chloride (PVC) layer deposited on a piece of polydimethylsiloxane (PDMS). Unexpectedly, the pickup/release temperatures were modified in wider temperature range simply by changing the thickness of the PVC layer than changing the plasticizer ratio. Utilizing the difference in the pickup/release temperatures depending on the PVC film thickness, 2D flakes were transferred from a thicker PVC film to a thinner one. This polymer-to-polymer transfer technique can be utilized to flip over van der Waals heterostructures. As a demonstration, we fabricated a mountain-like stacked structure of hexagonal boron nitride flakes using the flip-over stacking technique. Finally, we compared the results of thermomechanical analysis with the pickup/release temperatures of the PVC/PDMS stamp. The PVC was revealed to be at the glass transition and in the viscoelastic flow regimes when the 2D flakes were picked up and dry released, respectively. Our polymer-to-polymer transfer method facilitates flip-over van der Waals stacking in an all-dry manner, expanding the possibility of 2D materials device fabrications.


PVC/PDMS stamp preparation
Polyvinyl chloride (PVC) powder (degree of polymerization, 680) was provided by Shin-Etsu Chemical Co., Ltd. Plasticizer (Dioctyl Phthalate, DOP) was dropped into PVC powder (~0.5-1.0 g) placed in a glass bottle (Mity Vial 28 mL,No.6,Maruemu Corporation), and the PVC and DOP were then mixed using a plastic rod. The mixture was maintained at 25 °C overnight, and then cyclohexane (2-5 mL) was poured into the bottle. The bottle was heated to 100 °C on a hotplate and occasionally shaken to promote dissolution. The PVC was dissolved within 24 h.
Before use, the PVC solution was filtered by a syringe filter to remove particles. PVC films were then fabricated by one of the following two methods. The maximum thickness of the PVC films fabricated using method (1) was limited by the viscosity of the PVC solution (~15 µm). PVC films with thicknesses of up to ~150 μm were fabricated using method (2).
(1) Fabrication of PVC films using glass slides [ Fig. S1]: Tape was used to cover two of the edges of a glass slide to act as a spacer, and PVC solution was dropped onto the center of the uncovered area of the slide using a disposable polyethylene pipet (DISPET 0.5 mL, Maruemu Corporation).
The solution was spread over the glass using another glass slide. The PVC film was heated on a hotplate to T = 80 °C for more than 15 min for solidification. To increase the film thickness, the thickness of the spacer was adjusted by adding more layers of tape.
(2) Fabrication of PVC films using a tape frame structure [ Fig. S2]: Multiple layers of tape were attached to a glass slide, and the center of the tape was cut out to create a tape frame structure.
PVC solution was poured inside the frame and solidified on a hotplate.
A polydimethylsiloxane (PDMS) dome was prepared using a PDMS elastomer kit (SYLGARD ® 184, Dow Corning). A polymeric base and curing agent (10:1 by weight) were mixed and placed in a vacuum desiccator for 10 min to eliminate air bubbles. The liquid mixture was dropped on a PDMS sheet (PF-X4, 17 mm, Gel-Pak) that had been placed on a glass slide using a toothpick. The diameter of the dome was ~1-2 mm. The PDMS dome was solidified on a hotplate at T = 130 °C for 5 min.
To combine the PDMS dome and PVC film, a piece of tape was attached to each of the four edges of the PVC film, creating a PVC film window of ~4 × 4 mm 2 . The PVC film was then 3 peeled off from the glass slide and transferred to a PDMS dome on a glass slide [ Fig. S3]. Excess tape was trimmed using a rotary cutter. The PVC/PDMS stamp was baked in a vacuum oven at T = 80 °C overnight before use.

Comment on polymer-to-polymer transfer from thinner to thicker films
The transfer ratio was not zero even when transfer from thinner to thicker films was attempted, and the transfer ratio increased with temperature (~30% at T > 130 °C). It is possible that gravity or a temperature gradient across the PVC stamp was to some extent, responsible for this behavior. Expansion of the PDMS dome at high temperatures may also play a role.
6 Notes on the conditions required for successful polymer-to-polymer transfer The optimal thickness and DOP content of the PVC films used in polymer-to-polymer transfer are now discussed, along with some other practical aspects of the technique.
(1) Film Thickness: The PVC film needs to be reasonably thick to prevent deformation upon being brought into contact with the PVC stamp. When the film was too thin (~1 μm), it was separated from the PDMS dome during stamp detachment. When this occurred, the transfer rate was drastically decreased. Therefore, we recommend using PVC films thicker than 10 μm.
(2) DOP content: We assumed that the optimal DOP content of the PVC film was ~20-40%, at which both strong adhesion and film flexibility was realized. When the DOP content was greater than 40%, the adhesion between the PVC and 2D flakes was somewhat reduced. Meanwhile, without the addition of DOP, the PVC lacked flexibility and tended to crack as the film thickness increased. Thus, although Tpickup and Trelease can be modulated by tuning the plasticizer content, it is preferable to change Tpickup and Trelease by varying the film thickness.
(3) Preservation of PVC stamp: The PVC stamp should be used just after being baked in a vacuum oven or stored in a vacuum desiccator. When the stamp was used after being left in the air for long periods of time, polymer-to-polymer transfer did not occur.
(4) Transfer temperature: Polymer-to-polymer transfer did not occur when the temperature was rapidly increased far above the optimal temperature. Higher temperatures did not always result in higher transfer ratios. In practice, the best approach is to start at a temperature slightly lower than the optimal temperature, and then gradually increase the stage temperature until the transfer occurs.
(5) Reuse of PVC stamp: The pickup force of the PVC stamp became a little weaker after repeated use of the PVC stamp. Thus, we recommend renewing PVC stamps on a regular basis to ensure reproducibility.